Light, Photon Energies and Atomic Spectra (III)
When light collides with atoms (or molecules) it can be absorbed; its energy (if absorbed) is completely consumed by the atom or molecule.
The electrons of the atom (or molecule) exist in energy levels, and can make transitions from a low to a higher energy level, whereupon the difference of energy of the atom is exactly equal to the energy of the photon absorbed.
If the transition is from a high to a low level, a photon of light is emitted. Its energy is equal to the change in energy of the atom’s electrons in transiting from the high to the low energy level (of the atom).